This invention relates to a process for production of ZrB.sub.2 powder that is shaped into sintered refractory hard metal (RHM) bodies for use as cathode bodies in the aluminum industry.
Cathode bodies produced from available RHM powders have had unacceptably short service lifetimes because of intrinsic flaws traceable to carbon and oxygen impurities, phase inhomogeneity, microcracks and residual stresses. Impurities and phase inhomogeneity are carried into the cathode bodies from the RHM powder. Carbon and oxygen impurities are traceable to the raw materials while phase inhomogeneity arises from inadequate control of the powder production process. Microcracks and residual stresses form in RHM bodies during the densification process because of pore coarsening and/or shape change and lack of grain boundary pinning at the pores. Therefore, microcracks and residual stresses in finished parts can be minimized by powders that can give high green density parts, contain no foreign particles in the green state and have a narrow particle density range in the green state. Particles which can fulfill these requirements are those which are made from raw materials containing no oxygen or carbon; are processed with close temperature and composition controls; contain no foreign material; have a narrow particle size distribution with a very fine average particle size; and are equiaxed.
Processes for producing ZrB.sub.2 powder in a molten salt bath have been described in the prior art. However, each of the known prior art molten salt bath processes suffers from one or more serious disadvantages making it less than entirely suitable for its intended purpose.
For example, British Pat. No. 901,402 published July 18, 1962 includes an Example 13 wherein ZrB.sub.2 powder is formed by reaction of ZrO.sub.2 powder with Ca and CaB.sub.4 O.sub.7 in a molten medium comprising anhydrous CaCl.sub.2. The molten reactants are contained in a reactor maintained at a temperature of 870.degree.-910.degree. C. The product recovered included 94% ZrB.sub.2 powder having an average particle size of 20-35 millimicrons.
Some related processes for formation of RHM powders in a molten salt bath are disclosed in Meadows et al U.S. Pat. Nos. 3,520,656 and 3,692,479. Example 19 of the latter patent is a process for the preparation of a solid solution of TiB.sub.2 and ZrB.sub.2. Raw materials used are a mixture of TiO.sub.2, ZrO.sub.2, CaB.sub.4 O.sub.7 and Ca reacted together in a CaCl.sub.2 melt maintained at a temperature of 950.degree.-1000.degree. C. The resulting product is a solid solution of TiB.sub.2 and ZrB.sub.2 having a crystal size of about 40 millimicrons and 94% purity. Neither oxygen levels nor phase homogeneity was reported, and particle symmetry information is lacking.
Powders produced in reactions described in the above-mentioned patents suffer from having such fine particle size that, if pure, they are highly pyrophoric and are nearly impossible to handle in air without being at least partially oxidized. Without information on particle morphology, it is almost impossible to predict the suitability of such powders for end use as cathode bodies. In addition, because these diboride powders are made from metal oxide starting materials, they fall short of the high purity levels needed for successful production of RHM cathode bodies having utility in the aluminum industry.